1. Field of the Invention
This invention pertains to trenchers, that is, motorized devices for digging trenches up to ten feet or more deep, and six feet or more wide, at a single pass. Specifically, this invention pertains to improvements in the transmission and control of power to the cutting wheel and the tracks of the trencher.
2. Prior Art
The earliest trenching devices were undoubtedly the human hand, as man dug for food. Later, sharp sticks or rocks tied to sticks were used to cultivate the ground, transport water, or lay a trap for prey. Since then, however, the art of ditch digging has advanced considerably.
Today, for aesthetic, safety and other reasons, cables, power lines and pipes are laid underground, often spanning tremendous distances. Similarly, the depth and width of trenches necessary to accommodate the size pipes and other apparatus being laid in the ground can reach ten or more feet deep, and six or more feet wide. Further, the geological formations through which these trenches must be dug can range from soft loam to rock harder than concrete.
In any pipe laying project, the digging of the trench can be the most costly component in terms of money and time, as the pipe or cable can usually be laid as fast as the trench is dug. Therefore, increasing the speed of the trenching operation substantially lessens the cost of the overall project. Also, because these trenches will often traverse scenic areas, it is important that the trenching operation minimize its impact up the area as much as possible.
All of these factors have led to the development of the modern-day trenching machines. These devices are typically heavy duty industrial vehicles (such as wheeled or tracked vehicles) to which a rotating cutting wheel is appended. In some trenchers, the cutting wheel is elongated, resembling the blade of a chain saw. The cutting wheel is equipped with a number of buckets about its outer periphery, with each of the buckets having a number of cutting teeth. As the cutting wheel is rotated, the teeth cut into the ground, filling each of the individual buckets with the cut material. The buckets carry the material out of the trench and deposit their load onto a running conveyor belt which transports the material to the side of the trench. As the cutting wheel rotates, the vehicle travels ahead as fast as the material can be cut and removed from the trench.
In the trenching operation, the operator of the trencher has control over two essential variables. The first is the speed at which the trencher is set to move forward. The typical large scale trencher is equipped with a pair of tracks which have independent controls called "crowd handles". The operator, of course, wants the maximum forward speed which can be obtained. This will depend upon the terrain to be traversed and the material to be trenched. The appropriate setting on the crowd handles is arrived at through trial and error. Once set at an optimum position, the operator prefers not to change the setting. The second variable is the amount of torque delivered to the cutting wheel. Again, the operator strives for an optimal setting wherein the torque is sufficiently high such that the cutting wheel will not be stopped upon encountering every obstruction, but not so high, that when a truly immoveable obstruction is encountered, which does stop the cutting wheel, damage will result to the downstream components.
As might be expected, considerable power must be transmitted to the cutting wheel to rotate it with sufficient force to cut properly. This is not a problem, however, as a sufficiently large engine can be provided for that purpose. A problem does arise, however, due to the fact that notwithstanding the tremendous force which can be transmitted to the cutting wheel due to the reduction from the 2150 RPM at which the engine is turning to the 8 to 11 RPM at which the cutting wheel in turning and the large momentum which the cutting wheel generates during the digging operation, different geological formations of varying hardness, or large rocks not visible to the operator, will be encountered. The trencher may not be able to cut its way through these formations at the speed encountered. Indeed, it is not at all unusual for a cutting wheel, ten feet in diameter, 3 feet wide and weighing 35,000 pounds to be instantaneously stopped because of some unseen obstruction in the ground. Accordingly, a direct connection between power source and cutting wheel is not possible. Some means must be provided which will allow for slippage in that situation, otherwise catastrophic damage to the equipment will result due to the large overall reduction and the multiplication of torque. Moreover, the loss so occasioned will be far in excess of the damage to the equipment, because "down time" results in virtual stoppage of the trenching project.
A number of solutions have been proposed for this problem. For example, a belt drive mechanism has been used. This has not proven as satisfactory as desired, because belts require frequent replacement, particularly when they become burnt when the cutting wheel unexpectedly is stopped as described above. Further, belts are difficult to tension properly.
Another device utilized has been ordinary pneumatic tires, one on either side of the cutting wheel. The tires are rotated much the same as in an automobile, and are brought against the cutting wheel under pressure. This mechanism is a substantial improvement over belt drive, but still does not work as well as desired because sufficient tensioning may not be possible and, as with the belt system, tensioning is also difficult.
Another device previously utilized as a tortional limiting means in large scale trenchers is the twin disk over-center clutch. In this device, power is transmitted to downstream components by virtue of two or more dry clutch plates which act upon one another frictionally. The clutch plates are brought together by means of an over center toggle arrangement which includes a fine thread adjusting collar. Typically, this type of device, when used in a large scale trencher, has multiple disks and is located immediately aft of the digging transmission. As a result, the tortional requirements of the device are quite high and require substantial friction. Still, the intent of the device is to limit the amount of torque which can be transmitted to downstream components, so a proper setting of the adjusting collar is critical to the proper functioning of this device. This device, however, has no means for measuring the resulting friction achieved at any particular setting of the adjusting collar. Only an experienced operator, who has developed a "feel" for the device, can be expected to consistently adjust the device properly. It is not unusal for an inexperienced operator to tighten the adjusting collar too much, resulting in catastrophic failure of downstream components. Even experienced operators have failed to properly set this device.
In addition to the difficulties encountered in properly adjusting this device, another inherent drawback arises when the cutting wheel encounters an obstruction which causes the clutch plates to turn relative to one another; that is, changing from a static to a dynamic state. When this occurs, two things happen. First, the coefficient of friction of the materials used on most clutch plates increases in the dynamic state. Therefore, the clutch plates, which are supposed to slip when the cutting wheel encounters an immoveable object, actually work against that purpose. Second, when the plates obtain a dynamic state, the plates become very hot very quickly. That heat not only causes the plates to expand, but also causes the toggle mechanism and adjustment collar to become hot and expand, all of which tend to increase the amount of friction between the plates.
The operator must immediately release the clutch mechanism so that the clutch plates no longer act on one another. Once the obstruction has been cleared, however, this prior art device will not permit for the clutch to be reengaged at the same adjusting collar position, as the prior collar setting is no longer correct. This is as a result of latent heat which is stored in the plates and surrounding metal components, and the loss of friction material from the plates. (The friction material is typically asbestos, which is an excellent heat insulator, which increases the problems associated with heat retention). Therefore, before digging again, the operator must manually change the setting of the adjustment collar. That new setting in turn becomes imprecise as the components cool. In the event more obstructions are encountered before the system cools, (which is the usual situation), heat build-up occurs which can result in warped and damaged plates.
Another prior art device was an oil drive mechanism. Because of the problems associated with the dry twin disk over-center clutch mechanism, some people in the field have turned to a hydraulic system for power transmission, relying on release valves to protect the downstream components in the event the cutting wheel hits some immovable obstruction. This system, however, is subject to three drawbacks. First, the system is sensitive to hydraulic oil contamination. As might be imagined, the trenching operation is typically a very dusty, dirty environment. Therefore, the chances of contamination are enhanced greatly. Second, on large scale equipment, the system becomes quite complex with as many as four pumps, four motors and associated components necessary to transmit the desired power. The failure at any point of this complex system is a failure of the entire system. Because of its complexity, any failure is extremely time consuming because of the difficulty in diagnosis, and the extensive nature of repairs. Often, hydraulic repairs can not be effected in the field, and require that the components be returned to the shop. Moreover, any repairs in the field entail opening the system to the harsh environment, leading to further contamination and further failures.
Thirdly, the hydraulic system was undesirable because of the large horse power loss associated with the system, problems of heat generation, and problems with cavitation in the pumps following release of the relief valves.
The prior art devices have also not provided for immediate release when an obstruction which causes the cutting wheel to stop is encountered. Even though the devices may slip as designed, the slippage still places a tremendously increased load on the downstream drive train. Unless immediately released, damage may occur. At a minimum, this increases wear and tear on the other components, leading to more frequent repair. In the past, with prior art devices, equipment failure once a week or more is not unusual.
Therefore, a need exists in the field of large scale trenching equipment for an improved trencher which will overcome the deficiencies of the prior art.